CMSC 313 COMPUTER ORGANIZATION & ASSEMBLY LANGUAGE PROGRAMMING LECTURE 11, SPRING 2013

Transcription

1 CMSC 313 COMPUTER ORGANIZATION & ASSEMBLY LANGUAGE PROGRAMMING LECTURE 11, SPRING 2013

2 TOPICS TODAY C Input/Output Characters & Strings in C Structures in C Project 4

3 C INPUT/OUTPUT

4 stdin, stdout, stderr C opens three input/output devices automatically: stdin The standard input device, usually your keyboard stdout The standard output device, usually your monitor stderr The standard error device, usually your monitor Some C library I/O functions automatically use these devices Adapted from Dennis Frey CMSC 313 Fall 2011

5 Formatted Console Output printf( ) outputs formatted text to stdout Example: printf( format, arg1, arg2, ); int n = 3 ; printf ( Value = %d\n, n) ; format is a string containing conversion specifications literals to be printed Adapted from Dennis Frey CMSC 313 Fall 2011

6 printf( ) conversions Conversions specifications begin with % and end with a conversion character. Between the % and the conversion character MAY be, in order A minus sign specifying left-justification The minimum field width A period separating the field width and precision The precision that specifies Maximum characters for a string Number of digits after the decimal for a floating point Minimum number of digits for an integer An h for short or an l (letter ell) for long man printf for more documentation. Adapted from Dennis Frey CMSC 313 Fall 2011

7 Common printf( ) Conversions %d print integer as a decimal number (base 10) %u print integer as unsigned number %s print string %f print double as a floating point number %x print integer in hexadecimal (base 16) %c print integer as ASCII character %p print pointer in hexadecimal (implementation dependent) Adapted from Dennis Frey CMSC 313 Fall 2011

8 printf( ) Examples int anint = 5678; double adouble = 4.123; #define NAME Bob /* what is the output from each printf( ) */ printf ( %d is a large number\n, anint); printf ( %8d is a large number\n, anint); printf ( %-8d is a large number\n, anint); printf ( %10.2f is a double\n, adouble); printf( The sum of %d and %8.4f is %12.2f\n, anint, adouble, anint + adouble); printf ( Hello %s\n, NAME); Adapted from Dennis Frey CMSC 313 Fall 2011

9 Formatted Output Example Use field widths to align output in columns int i; for (i = 1 ; i < 5; i++) printf("%2d %10.6f %20.15f\n", i,sqrt(i),sqrt(i)); Adapted from Dennis Frey CMSC 313 Fall 2011

10 Keyboard Input scanf reads user input from stdin. Syntax for scanf( ) is similar to printf( ) scanf( format, arg1, arg2,... ) The format string similar structure to printf( ). The arguments must be addresses of the variables. Adapted from Dennis Frey CMSC 313 Fall 2011

11 scanf( ) format string The scanf( ) format string usually contains conversion specifications that tell scanf( ) how to interpret the next input field. An input field is a string of non-whitespace characters. The format string usually contains Blanks or tabs which are ignored Ordinary characters which are expected to match the next (nonwhitespace) character input by the user Conversion specifications usually consisting % character indicating the beginning of the conversion An optional h, l (ell) or L A conversion character which indicates how the input field is to be interpreted. Adapted from Dennis Frey CMSC 313 Fall 2011

12 Common scanf( ) conversions %d a decimal (integer) number %u an unsigned decimal (integer) number %x a hexadecimal number %f a floating point number with optional sign, decimal point, and exponent %s a string delimited by white space, NOT an entire line %c a single character (possibly a whitespace char) Adapted from Dennis Frey CMSC 313 Fall 2011

13 scanf( ) examples int age; double gpa; char initial; printf( input your middle initial: ); scanf ( %c, &initial ); printf( Input your age: ); scanf( %d, &age ); printf( input your gpa: ); scanf ( %lf, &gpa ); // note & Adapted from Dennis Frey CMSC 313 Fall 2011

14 Unix I/O redirection Redirect input (read from infile instead of keyboard): a.out < infile Redirect output (write to outfile instead of screen): a.out > outfile Redirect both: a.out < infile > outfile Redirect stdout and stderr to outfile a.out >& outfile Redirect stdout to outfile and stderr to errfile (a.out > outfile) >& errfile Adapted from Dennis Frey CMSC 313 Fall 2011

15 Text File I/O Use fprintf() and fscanf() functions instead of printf() and scanf(). Must open file before reading/writing: fopen() Must close file after all done: fclose() Use file handle to specify file. File handle returned by fopen(): FILE *myfile ; myfile = fopen ( bob.txt, r ) ; if (myfile == NULL) { } /* handle the error */ Adapted from Dennis Frey CMSC 313 Fall 2011

16 fopen( ) fopen( ) requires two parameters 1. The name of the text file to be opened 2. The text file open mode r w a open the file for reading only create the file for writing; delete existing file append; open or create the file for writing at the end r+ open the file for reading and writing w+ a+ create the file for reading & writing; deletes existing file open or create the file for reading or writing at the end Adapted from Dennis Frey CMSC 313 Fall 2011

17 fscanf.c #include <stdio.h> #include <stdlib.h> /* for exit */ int main ( ) { double x ; FILE *ifp ; /* try to open the file for reading, check if successful */ /* if it wasn't opened exit gracefully */ ifp = fopen("test_data.dat", "r") ; if (ifp == NULL) { printf ("Error opening test_data.dat\n"); exit (-1); } fscanf(ifp, "%lf", &x) ; /* read one double from the file */ fclose(ifp); /* close the file when finished */ /* check to see what you read */ printf("x = %.2f\n", x) ; return 0; } Adapted from Dennis Frey CMSC 313 Fall 2011

18 Detecting end-of-file with fscanf When reading an unknown number of data elements from a file using fscanf( ), we need a way to determine when the file has no more data to read, i.e, we have reached the end of file. Fortunately, the return value from fscanf( ) holds the key. fscanf( ) returns an integer which is the number of data elements read from the file. If end-of-file is detected the integer return value is the special value EOF Adapted from Dennis Frey CMSC 313 Fall 2011

19 EOF example code /* code snippet that reads an undetermined number of integer student ages from a file and prints them out as an example of detecting EOF */ FILE *infile; int age; infile = fopen( myfile, r ); if (infile == NULL) { printf ("Error opening myfile\n"); exit (-1); } while ( fscanf(infile, %d, &age ) = EOF ) { printf( %d\n, age ); } fclose( infile ); Adapted from Dennis Frey CMSC 313 Fall 2011

20 fprintf.c #include <stdio.h> #include <stdlib.h> /* exit */ int main ( ) { double pi = ; FILE *ofp ; /* try to open the file for writing, check if successful */ ofp = fopen("test.out", w") ; if (ofp == NULL) { printf ("Error opening test.out\n"); exit (-1); } /* write to the file using printf formats */ fprintf(ofp, Hello World\n ); fprintf(ofp, PI is defined as %6.5lf\n, pi); fclose(ofp); /* close the file when finished reading */ return 0; } Adapted from Dennis Frey CMSC 313 Fall 2011

21 CHARACTERS & STRINGS

22 char type C supports the char data type for storing a single character. char uses one byte of memory. char constants are enclosed in single quotes char mygrade = A ; char yourgrade =? ;

23 ASCII Character Chart

24 Special Characters Use \ for escape sequences. For example \n is the newline character \t is the tab character \ is the double quote (necessary since double quotes are used to enclose strings \ is the single quote (necessary since single quotes are used to enclose chars \\ is the backslash (necessary since \ now has special meaning \a is beep which is unprintable

25 Special Char Example Code What is the output from these statements? printf( \t\tmove over\n\nworld, here I come\n"); Move over World, here I come printf("i\ ve written \ Hello World\ \n\t many times\n\a ); I ve written Hello World many times <beep>

26 Character Library Functions int isdigit (int c); Determine if c is a decimal digit ( 0-9 ) int isxdigit(int c); Determines if c is a hexadecimal digit ( 0-9, a - f, or A - F ) int isalpha (int c); Determines if c is an alphabetic character ( a - z or A- Z ) int isspace (int c); Determines if c is a whitespace character (space, tab, etc) int isprint (int c); Determines if c is a printable character int tolower (int c); int toupper (int c); Returns c changed to lower- or upper-case respectively, if possible

27 Character Library Functions Include header file use character library functions: #include <ctype.h> Technically functions take an int parameter, not char. Return type is also int. 0 = False, not 0 = True. man ctype.h for more functions and complete documentation.

28 Character Input/Output Use %c in printf( )and fprintf( )to output a single character. char yourgrade = A ; printf( Your grade is %c\n, yourgrade); Input char(s) using %c with scanf( ) or fscanf( ) char grade, scores[3]; %c inputs the next character, which may be whitespace scanf( %c, &grade); %nc inputs the next n characters, which may include whitespace. scanf( %3c, scores); // note -- no & needed

29 Strings in C String = null terminated array of char. null = '\0' String constants in double quotes are null terminated. Strings do not "know" their own length. Initialization: char name4[ 20 ] = { B, o, b, b, y, \0 }; char name5[6] = Bobby ; // NOT assignment, needs 6 slots char name6[ ] = Bobby ;

30 String Output char name[ ] = Bobby Smith ; printf( My name is %s\n, name); // Right and left justify printf ( My favorite books are %12s and %12s\n, book1, book2); printf ( My favorite books are %-12s and %-12s\n, book1, book2);

31 Dangerous String Input char name[22]; printf( Enter your name: ); scanf( %s, name); Why is this dangerous? Long name will overwrite memory.

32 Safer String Input char name[ 22 ]; printf( Enter your name: ); scanf( %21s, name); // note 21, not 22, 1 byte for '\0'

33 C String Library C provides a library of string functions. To use the string functions, include <string.h>. Some of the more common functions are listed here on the next slides. To see all the string functions, type man string.h at the unix prompt.

34 C String Library (2) Must #include <string.h> strlen( const char string[ ] ) Returns length of string, not counting '\0' strcpy( char s1[ ], const char s2[ ] ) Copies s2 on top of s1. Must have enough space in s1 The order of the parameters mimics the assignment operator strcmp ( const char s1[ ], const char s2[ ] ) Returns < 0, 0, > 0 if s1 < s2, s1 == s2 or s1 > s2 lexigraphically strcat( char s1[ ], const char s2[ ]) Appends (concatenates) s2 to s1. Must have enough space in s1

35 C String Library (3) Some safer functions from the C string library: strncpy( char s1[ ], const char s2[ ], int n ) Copies at most n characters of s2 on top of s1. Does not null terminate s1 if length of s2 >= n The order of the parameters mimics the assignment operator strncmp ( const char s1[ ], const char s2[ ], int n ) Compares up to n characters of s1 with s2 Returns < 0, 0, > 0 if s1 < s2, s1 == s2 or s1 > s2 lexigraphically strncat( char s1[ ], const char s2[ ], int n) Appends at most n characters of s2 to s1.

36 String Code char first[10] = bobby ; char last[15] = smith ; char name[30]; char you[ ] = bobo ; strcpy( name, first ); strcat( name, last ); printf( %d, %s\n, strlen(name), name ); strncpy( name, last, 2 ); printf( %d, %s\n, strlen(name), name ); int result = strcmp( you, first ); result = strncmp( you, first, 3 ); strcat( first, last );

37 Simple Encryption char c, msg[] = "this is a secret message"; int i = 0; char code[26] = /* Initialize our encryption code */ {'t','f','h','x','q','j','e','m','u','p','i','d','c', 'k','v','b','a','o','l','r','z','w','g','n','s','y'} ; printf ("Original phrase: %s\n", msg); /* Encrypt */ while( msg[i] = '\0 ){ if( isalpha( msg[ i ] ) ) { c = tolower( msg[ i ] ) ; msg[ i ] = code[ c - a ] ; } ++i; } printf("encrypted: %s\n", msg ) ;

38 Arrays of Strings An initialized array of string constants char months[][4] = {"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" } ; int m; for ( m = 0; m < 12; m++ ) printf( %s\n, months[ m ] ); Alternative: use typedef typedef char Acronym[4] ; Acronym months[] = {"Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" } ;

39 sprintf( ) sprintf( ) works just like printf( ) or fprintf( ), but puts its output into the specified character array. The character array must be big enough. char message[ 100 ]; int myage = 4; sprintf( message, I am %d years old\n, age); printf( %s\n, message);

40 STRUCT

41 Java vs C Suppose you were assigned a write an application about points and straight lines in a coordinate plane. In Java, you d correctly design a Point class and a Line class using composition. What about in C?

42 No Classes in C Because C is not an OOP language, there is no way to combine data and code into a single entity. Related data and functions are form an "Abstract Data Type." Accessibility is enforced by a programmer's good judgment and not by the compiler. C does allow us to combine related data into a structure using the keyword struct. All data in a struct variable can be accessed by any code. Think of a struct as an OOP class in which all data members are public, and which has no methods.

43 Struct definition struct tag { member1_declaration; member2_declaration; member3_declaration;... membern_declaration; }; semi-colon struct is the keyword tag names this kind of struct, member_declarations are variable declarations which define the data members.

44 C struct Example Defining a struct to represent a point in a coordinate plane struct point point is the struct tag { int x;/* x-coordinate */ int y;/* y-coordinate */ }; Given the declarations struct point p1; struct point p2; we can access the members of these struct variables: the x-coordinate of p1 is p1.x the y-coordinate of p1 is p1.y the x-coordinate of p2 is p2.x the y-coordinate of p2 is p2.y

45 Using struct members int main ( ) { struct point lefeendpt, rightendpt, newendpt; printf( Left end point cooridinates ); scanf( %d %d, &lefeendpt.x, &leftendpt.y); printf( Right end point s x-coordinate: ); scanf( %d %d, &righteendpt.x, &rightendpt.y); // add the endpoints newendpt.x = leftendpt.x + rightendpt.x; newendpt.y = leftendpt.y + rightendpt.y; // print new end point printf( New endpoint (%2d, %2d), newendpt.x, newendpt.); } return 0;

46 Initializing a struct struct point middle = { 6, -3 }; is equivalent to struct point middle ; middle.x = 6 ; middle.y = -3 ;

47 struct Variants struct point { int x, y; } endpoint, upperleft ; defines the structure named point AND the variables endpoint and upperleft to be of this type.

48 struct + typedef typedef struct point { int x, y; } POINT; POINT is now a TYPE. POINT endpoint ; is equivalent to struct point endpoint;

49 struct assignment struct point p1; struct point p2; p1.x = 42; p1.y = 59; p2 = p1; /* structure assignment copies members */

50 struct within a struct typedef struct line { POINT leftendpoint; POINT rightendpoint; } LINE; LINE line1, line2; line1.leftendpoint.x = 3 ; line1.leftendpoint.y = 4 ;

51 Arrays of struct LINE lines[5]; // or struct line lines[5]; printf("%d\n", lines[2].leftendpoint.x);

52 Arrays within a struct Structs may contain arrays as well as primitive types struct month { int nrdays; char name[ ]; }; struct month january = { 31, JAN };

53 A bit more complex struct month allmonths[ 12 ] = { {31, JAN }, {28, FEB }, {31, MAR }, {30, APR }, {31, MAY }, {30, JUN }, {31, JUL }, {31, AUG }, {30, SEP }, {31, OCT }, {30, NOV }, {31, DEC } }; // write the code to print the data for September printf( %s has %d days\n, allmonths[8].name, allmonths[8].nrdays); // what is the value of allmonths[3].name[1]

54 Size of a struct As with primitive types, we can use sizeof( ) to determine the number of bytes in a struct int pointsize = sizeof( POINT ); int linesize = sizeof (struct line); As we ll see later, the answers may surprise you

55 NEXT TIME Parameter passing Separate Compilation Scope & Lifetime